core/legal-info: ensure legal-info works in off-line mode

core/legal-info: ensure legal-info works in off-line mode

Almost all packages which are saved for legal-info have their source
archives downloaded as part of 'make source', which makes an off-line
build completely possible [0].

However, for the pre-configured external toolchains, the source tarball
is different, as the main tarball is a binary package. And that source
tarball is only downloaded during the legal-info phase, which makes it
inconvenient for full off-line builds.

We fix that by adding a new rule, $(1)-legal-source which only
$(1)-all-source depends on, so that we only download it for a top-level
'make source', not as part of the standard download mechanism (i.e. only
what is really needed to build).

This new rule depends, like the normal download mechanism, on a stamp
file, so that we do not emit a spurious hash-check message on successive
runs of 'make source'.

This way, we can do a complete [0] off-line build and are still able to
generate legal-info, while at the same time we do not incur any download
overhead during a simple build.

Also, we previously downloaded the _ACTUAL_SOURCE_TARBALL when it was
not empty. However, since _ACTUAL_SOURCE_TARBALL defaults to the value
of _SOURCE, it can not be empty when _SOURCE is not. Thus, we'd get a
spurious report of a missing hash for the tarball, since it was not in
a standard package rule (configure, build, install..) and thus would
miss the PKG and PKGDIR variables to find the .hash file.

We fix that in this commit as well, by:

  - setting PKG and PKGDIR just for the -legal-source rule;

  - only downloading _ACTUAL_SOURCE_TARBALL if it is not empty *and* not
    the same as _SOURCE (to avoid a second report about the hash).

[0] Save for nodejs which invarriably wants to download stuff at build
time. Sigh... :-( Fixing that is work for another time...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Tested-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>